Moscow -- - A day after the Russian leader Vladimir Putin proposed to President Obama that they boost attempts to resolve their standoff over Ukraine, Secretary of State John Kerry scrambled his travel plans to meet with his Russian counterpart Sunday in Paris.

The meeting comes amid fears that Russia plans to seize more Ukrainian territory after its recent annexation of Crimea that led to U.S. and European sanctions.

As the tug-of-war over Ukraine's future continued, Vitali Klitschko - one of the best-known faces of the antigovernment rallies that helped set off the country's political crisis - threw his support to a competitor for the presidency in hopes of unifying forces behind a single, pro-Western candidate.

The announcement by Klitschko, a former world champion boxer, that he would put aside his presidential ambitions in favor of billionaire Petro Poroshenko reordered the race ahead of elections in May. The move appeared to reflect rising concern of a split in support among candidates who want closer relations with the West, including the former prime minister, Yulia Tymoshenko, that could create an opening for a pro-Russia challenger.

"The presidential elections in Ukraine on May 25 should join society and not become another war of everyone against everyone," Klitschko said at a meeting of his party, the United Democratic Alliance for Reform.

Klitschko said he would run instead for mayor of Kiev, with a goal of changing the city into a "truly European capital."

The months of demonstrations, which eventually toppled Viktor Yanukovych, the president at the time, centered on whether Ukraine would tilt more toward Moscow or the West and eventually spilled over into the worst strains between Russia and the United States and its allies since the end of the Cold War.

On Saturday, in an apparent bid to defuse those tensions, the Russian foreign minister, Sergei Lavrov, said in a television interview that Russia had "no intention" of invading Ukraine, although the United States and NATO have said Russian forces were massed along the Ukrainian border.

Lavrov and Kerry spoke by telephone Saturday after Obama and Putin had agreed on fresh diplomacy. Kerry then delayed his return to the United States and headed for Paris to meet Lavrov on Sunday.